Heat-sensitive recording materials using a color-forming reaction between a leuco dye and a developer are relatively inexpensive, and the recording apparatuses are compact and easily maintained. Therefore, heat-sensitive recording materials are used not only as recording media for facsimiles, word processors, a variety of computers and the like, but also as recording media for medical instruments for ultrasonic diagnoses, X-ray image diagnoses, NMR (MRI) tomographic image diagnoses, etc.
When such heat-sensitive recording materials are used as recording media for medical diagnoses, an extremely slight density difference in an image is taken as information and used for diagnosis, and therefore image failures such as density unevenness, and omission of pixels (dots) generated during recording should be reduced as much as possible. Generally, a schaukasten is used for diagnoses with medical diagnosis images. Therefore, surroundings (background) of human body images, etc., which are diagnostic objects, are required to have a high black color density so as to block the light from a schaukasten and to prevent unnecessary light from blinding a diagnosing person's (doctor's) eyes. For these reasons, thermal printers used for imaging diagnoses are designed such that the energy width necessary to obtain a saturated transmission density (DT-max), i.e., the dynamic range, is wide so as to suppress density unevenness problems, etc. that are caused by the slight heat conduction differences between heating resistors in the thermal head, and that recording media are made to color at a high saturated transmission density. Therefore, compared to ordinary facsimiles, label printers, etc., thermal printers used for imaging diagnoses have a longer thermal-head-heating time (pulse width) during recording, and an extremely high amount of thermal energy is applied during recording. Therefore, compared to heat-sensitive recording materials used for facsimiles and label printers, heat-sensitive recording materials used for medical diagnoses have disadvantages in terms of residue adhesion to a thermal head and thermal-head wear.
In order to reduce residue adhesion to a thermal head, as well as thermal-head wear, the following heat-sensitive recording materials have been proposed; a heat-sensitive recording material comprising long-chain alkyl ether-modified polyvinyl alcohol in a protective layer (see Patent Document 1); a heat-sensitive recording material comprising a silicon-modified polymer and inorganic ultra fine particles having a mean primary particle diameter of 0.1 μm or less in a protective layer (see Patent Document 2); a heat-sensitive recording material comprising a silane-modified polymer in a protective layer (see Patent Document 3); a heat-sensitive recording material having a surface roughness (Ra value) of the recorded surface after recording of 0.7 μm or less when recording was conducted with an applied energy of 120 mJ/m2 (see Patent Document 4); a heat-sensitive recording material comprising microparticle-aggregation particles in the outermost layer of the recording side (see Patent Document 5); a heat-sensitive recording material comprising a pigment having a 50% volume average particle diameter of 0.25 to 0.40 μm measured by using a laser diffraction method, and the content of particles having a particle diameter of 1.0 μm or more based on all of the particles being more than 3.0 mass % and 9.0 mass % or less in a protective layer (see Patent Document 6); and a heat-sensitive recording material comprising a protective layer containing a water-soluble resin and calcined clay (namely, calcined kaolin) and formed on a heat-sensitive coloring layer, wherein the calcined clay is present in a proportion of at least 10 mass % of the protective layer to improve chemical resistance and head-matching property (see Patent Document 7). However, heat-sensitive recording materials that possess satisfactory required features have not been obtained yet. Further, a heat-sensitive recording material comprising kaolin having a volume mean particle diameter of 0.8 μm in a protective layer is also known (see Patent Document 8).
Patent Document 1: Japanese Unexamined Patent Publication No. 2000-118133 (Claim 1)
Patent Document 2: Japanese Unexamined Patent Publication No. 2000-118138 (Claim 1)
Patent Document 3: Japanese Unexamined Patent Publication No. 2000-238432 (Claim 1)
Patent Document 4: Japanese Unexamined Patent Publication No. 2000-355165 (Claim 1)
Patent Document 5: Japanese Unexamined Patent Publication No. 2002-086911 (Claim 1)
Patent Document 6: Japanese Unexamined Patent Publication No. 2003-251936 (Claim 1)
Patent Document 7: Japanese Unexamined Patent Publication No. 1990-070483 (Claim 1)
Patent Document 8: International Publication WO2004/024460 (Example 1)